Digital or digitized signal processing has always been sensitive to signal baseline shifts whenever a digital or bipolar signal was unbalanced toward one polarity or the other, the shift toward the dominant polarity brings the baseline toward that polarity.
Digital waveforms (also known as binary signals), such as the NRZI pattern 16 shown in FIG. 1, have no baseline shift, which means that are only two levels in that waveform. The waveform has a high level denoted HI, a low level denoted LO and transitions between two levels denoted T. Idealized reproduction of a digital waveform requires a noiseless and infinite bandwidth linear analog channel.
When a digital waveform passes through an analog channel, such as a magnetic tape write/read channel or an optical disk write/read channel, it experiences distortions.
Distortions are caused mainly by limited bandwidth of the channel and the addition of noise. High frequency limits or high frequency roll-offs of the channel cause high frequency (or short wavelength) signals to have less amplitude than low frequency (or long wavelength) signals. Waveform or signal 18 illustrates high frequency roll-off distortions. AC coupling employed in channels to avoid DC shifts and drifts between different stages of signal processing circuits causes removal of the DC component of the signal and hence baseline shift. A baseline is an imaginary line connecting average points between two adjacent opposite peaks of an analog waveform. Baseline shift is illustrated by numeral 19. Depending on wavelength and digital sequence, both high frequency roll-off and AC coupling cause analog signals to shift the baseline. Additional causes of the base line movement of a read back signal in an optical disk channel are changes in media reflectivity and birefringence of the optical disk substrate. Because of the above reasons, the peaks and transitions in analog waveforms occur at different, often unpredictable, levels and are difficult to reliably detect by conventional fixed threshold detectors.
In data detectors, baseline shift can cause an apparent shift in transition positions (zero axis- crossing shifts with baseline shifts). One way to accommodate or prevent some of the baseline shifts is to provide balanced codes; i.e., the amount of signal in one polarity is equal to the same amount of signal in the opposite polarity, such that the net DC component of the signal is zero. This increases the overhead of the recorded data, therefore, it is desirable to provide a data detection system which rapidly compensates for any DC shift in signal baseline.
The simplest way to detect analog data transitions, which are carrying digital information, for example, is to compare the amplitude of the analog waveform with a fixed amplitude threshold. In such detection, each time the analog signal amplitude crosses the threshold, a transition is indicated. The slope of the analog transition; i.e., at the zero axis crossing, depends on many factors, including the read-channel bandwidth and its characteristics. Generally, the lower the bandwidth, the lower the slope. The lower the slope, the more difficult it is to obtain a precise location of the zero axis crossing, as is necessary in detecting pulse-width modulated signals. Also, it is well known that the analog-signal baseline (DC level) is constantly moving. The real crossing point between the transition and the fixed threshold will shift causing data detection errors. If the analog-signal moves entirely above or below the fixed threshold, the transition detection fails. The optical disk baseline movements of a readback signal, which is turned to analog signal, shifts toward either polarity and dynamically changes because of the often-used alternating current (AC) coupling, changes in media reflectivity and by birefringence of the substrate through which a reading or sensing laser beam passes to reach the recording layer of the optical medium.